Here's how to make a dust catcher for your easel using aluminium foil:
Use a piece of foil that's slightly longer than the bottom edge of your backing board. Fold the aluminium in half (the long way) so that the dull side, which is less reflective, faces out.
Bend (the long way again) so it forms a 90 degree corner that will sit on the shelf of your easel. It can be secured to the easel with tape if you like.
When the backboard is placed on the easel, part of the foil will be behind the backboard, the other part will sit under the board to catch any dust that falls.
Once the board is in place, fold the edges of the aluminium up to form a tray so the dust doesn't slide off the edges.
You can make a similar tray using paper, but I like using foil because it can be reused longer. The dust slides off easily when tipped out. The foil can then be brushed off or even wiped with a damp cloth, and it's good as new.